This invention relates to a process of isotope separation and more particularly to a process in which .sup.15 N isotopes are selectively extracted from natural abundance nitric oxide (NO).
At present .sup.15 N isotopes, available from commercial suppliers, are prepared by a distillation process from the nuclear generation of daughter product in fission reactions. A mole of .sup.15 NO at 99.5% purity made using this technique costs about $15,000, and is not likely to become available at either a significantly lower price or in a significantly greater amount in the foreseeable future. As there is a continuing demand for the .sup.15 N isotope, as well as other light isotopes, particularly in the nuclear-power industry, there is considerable impetus for finding a much larger, more economical process for producing them.
A technique of .sup.15 N or any other light-isotope preparation should be based on a separation of the desired species from the element found in its natural abundance. Otherwise, both the volume to be produced and the subsequent cost will be inadequate for commercial purposes. .sup.15 N isotopes comprise 0.37% of natural abundance nitrogen, and hence should be capable of separation in single-stage processes which display a single-step enrichment factor of several hundred.
The discovery of the laser with its many diverse photochemical applications has offered great new possibilities to photochemists. Recently developed excimer lasers have effectively opened up new regions of the spectrum to photochemists, since they operate in the ultraviolet and vacuum ultraviolet. These lasers are 1%-3% efficient (based upon wall plug energy) and have a narrow (10A) free-oscillating bandwidth. In addition, the excimer lasers may be frequency-tuned with intracavity dispersive elements to obtain very narrow linewidth with high peak powers (.gtoreq. 1Mwatt).
It has been known for many years that NO absorbs radiation in the 1600-2300A region is strong discrete absorptions corresponding to several low-lying electronic transitions. Photochemical measurements have shown that NO excited to the A (.sup.2 .SIGMA..sup.+) or B (.sup.2 .pi.) states will undergo bimolecular reactions with other species, such as ground state NO, CO.sub.2 and many other hydrocarbons.
However, several problems confront the photochemist in designing an effective process for an isotopic separation. First, the desired isotopic product molecules, such as .sup.15 NO, must be capable of a selective excitation in a natural abundance mixture with other isotopic molecules, such as .sup.14 NO. The desired molecules, after being selectively electronically excited, must react in a bimolecular process which yields stable reaction products. Also, unwanted side reactions, such as .sup.15 NO+NO, in the case of NO, and other radical chain reactions which scramble the selectivity, must be supressed. The desired products of the reaction have to be efficiently removed from the reaction zone without undergoing secondary reactions, and finally, the entire process must be cost effective so as to compete with currently used techniques.